1. Field of the Invention
The present invention relates to a television tuner, and, more particularly, to a television tuner in which the operation switching of the attached option circuit is simple.
2. Description of the Related Art
FIG. 2 shows the primary portion of a conventional television tuner. A mixer 42 and an intermediate frequency amplifier 43 are included within an integrated circuit 41. The mixer 42 is composed of a balanced circuit, and the balanced output ports thereof are connected to two output terminals 41a and 41b provided in the integrated circuit 41. Also, a selected television signal and a local oscillating signal are input to the mixer 42 and the intermediate frequency signal is output to the output terminals 41a and 41b. The intermediate frequency amplifier 43 is composed of a balanced circuit, and the balanced input ports thereof are connected to two input terminals 41c and 41d provided in the integrated circuit 41.
An intermediate frequency tuning circuit 44 provided outside of the integrated circuit 41 is composed of a parallel tuning circuit comprised of a capacitive element 44a and serially connected inductance elements 44b and 44c, and is connected between the output terminals 41a and 41b. Also, the power supply voltage Vcc applied to the connection point of the two inductance elements 44b and 44c is fed to the mixer 42. In addition, two parallel resonance circuits 45 and 46 are provided outside of the integrated circuit 41. The first parallel resonance circuit 45 is coupled between the output terminal 41a and the input terminal 41c, and the second parallel resonance circuit 46 is coupled between the output terminal 41b and the input terminal 41d. 
In the above-described construction, the intermediate frequency tuning circuit 44 tunes at the intermediate frequency band, the first parallel resonance circuit 45 resonates at a video intermediate frequency of a different channel adjacent to the intermediate frequency band, and the second parallel resonance circuit 46 resonates at an audio intermediate frequency of a different channel adjacent to the intermediate frequency band. Thereby, the interference from the adjacent channel is eliminated (For example, refer to Japanese unexamined Patent Publication No. 2003-153107 (FIG. 3))
In such a television tuner, in addition to the interference from the adjacent channel, an FM trap circuit may be provided in the input portion of the tuner (the front stage of the mixer) so as to attenuate a FM broadcasting band in order to avoid the interference due to, for example, the FM broadcasting signal, or an attenuator may be provided to attenuate the strong electric field television signal in order to prevent the distortion generated due to the strong electric field television signal. However, since it is needed to be switched such that the FM trap circuit or the attenuator is not operated in accordance with the usage purpose, a switch control circuit must be provided in the integrated circuit and the operation of the FM trap circuit or the attenuator must be switched according to the switch voltage output from the switch control circuit.
FIG. 3 shows the construction for switching the operation of an option circuit such as the FM trap circuit or the attenuator. Used for connecting an antenna, terminal 21 is connected in cascade with two option circuits 22 and 23 each having a different function. The first option circuit 22 is, for example, an attenuator of which the attenuated amount can be switched. The first option circuit 22 is switched to attenuate the received signal when the strength of the electric field of the received television signal is large, and is switched not to attenuate the signal when the electric field thereof is small. The second option circuit 23 is, for example, an FM trap circuit. The second option circuit 23 is switched to attenuate the FM broadcasting band by the trap function in case of receiving the television signal of the ground wave, and is switched to invalidate the trap function in the case of receiving the CATV television signal.
An input tuning circuit, a high frequency amplifier, and the interstage tuning circuit (not shown) are connected in cascade between the second option circuit 23 and the balanced mixer 24. The mixer 24 is supplied with a local oscillating signal (L.O) from an oscillator (not shown). The output side of the mixer 24 is connected with an intermediate frequency tuning circuit 25 and an adjacent channel trap circuit 26. The balance type intermediate frequency tuning circuit 25 is composed of a parallel tuning circuit comprised of a capacitive element 25a and two serially connected inductance elements 25b and 25c, which are connected in parallel to the capacitive element 25a, and it tunes at the frequency close to the center of the intermediate frequency band. In addition, the parallel tuning circuit is connected in parallel with a resistor 25d in order to adjust the Q of the tuning adequately.
Further, the power supply voltage Vcc applied to the connection point of the two inductance elements 25b and 25c is supplied to the mixer 24 through the two inductance elements 25b and 25c. The adjacent channel trap circuit 26 is comprised of a parallel resonance circuit formed of an inductance element 26a and a capacitive element 26b, and attenuates the video signal of the intermediate frequency band corresponding to the channel adjacent to the high frequency to eliminate the interference.
The integrated circuit 20 comprises an intermediate frequency amplifier 27, a bias circuit 28 and a switch control circuit 29. The intermediate frequency amplifier 27 is composed of a balanced circuit. In addition, the integrated circuit 20 is provided with parallel input terminals 20a and 20b, a power supply terminal 20c, and two switch terminals 20d and 20e. Also, one end of the intermediate frequency tuning circuit 25 is AC-connected to the input terminal 20a through the adjacent channel trap circuit 26, and the other end thereof is AC-connected to the input terminal 20b. The power supply terminal 20c is supplied with the power supply voltage Vcc.
In the integrated circuit 20, the input port of the intermediate frequency amplifier 27 is connected to the parallel input terminals 20a and 20b. In addition, the bias circuit 28 generates a bias voltage from the power supply voltage Vcc to supply it to the input port of the intermediate frequency amplifier 27. Thereby, the intermediate frequency signal output from the mixer 24 is input from the input terminals 20a and 20b to the intermediate frequency amplifier 27 through the intermediate frequency tuning circuit 25 and the adjacent channel trap circuit 26 and it is amplified therein.
The switch control circuit 29 has two switch transistors 29a and 29b, of which the collector is applied with the power supply voltage Vcc. The emitter of the switch transistor 29a is connected to the switch terminal 20d, and the emitter of the switch transistor 29b is connected to the switch terminal 20e. Moreover, each of the bases is applied with a switch signal (voltage) for turning on/off each of the switch transistors 29a and 29b, thereby the switch voltage is output from each emitter to the switch terminals 20d and 20e. 
The switch voltage of the switch terminal 20d is supplied to the second option circuit 23 through two serially connected resistors 30 and 31. The connection point of the two resistors 30 and 31 is high-frequency-grounded by a capacitive element 32. Similarly, the switch voltage of the switch terminal 20e is supplied to the first option circuit 22 through the two resistors 33 and 34. The connection point of the two resistors 33 and 34 is high-frequency-grounded by a capacitive element 35.
In the above-mentioned construction, upon operating the first option circuit 22 to attenuate the television signal, the switch transistor 29b is turned on and the switch voltage output from the emitter thereof is applied to the first option circuit 22. Similarly, upon operating the second option circuit 23 to attenuate the FM broadcasting band, the switch transistor 29a is turned on and the switch voltage output from the emitter thereof is applied to the second option circuit 23. When each of the option circuits 22 and 23 is not operated, the switch transistors 29a and 29b are turned off.
Several switch signals among the switch signals for switching the operation of the option circuit are output from the integrated circuit. Hence, the integrated circuit is provided with a terminal for outputting the switch voltage. Generally, if the number of integrated circuits is increased, the package size becomes increased and the scale of the circuit becomes increased, Hence increasing the manufacturing cost thereof. In addition, the option circuit is not necessarily needed, and is provided based in the type of the receiver used in the television tuner. Thereby, if it is planned to have one common integrated circuit, the integrated circuit corresponding to the option circuit is used even in the case where it is not necessary to include the option circuit, hence further increasing the manufacturing cost thereof.